Crucilble and method of growing single crytal by using crucible

ABSTRACT

Provided are a crucible which prevents polycrystal formation to easily allow growth of optical part material single crystals, and a single crystal growth method employing the crucible.  
     The crucible has a smooth surface of about Rmax 3.2s as the surface roughness of the wall surface  1 H, concave curved plane  1 J, cone surface  1 F and convex curved plane  1 L of the starting material carrying section  1 D and the wall surface  1 K of the seed carrying section  1 E, which constitute the inner surface of the crucible of a crucible body  1 A.

TECHNICAL FIELD

The present invention relates to a crucible for growth of a singlecrystal by melting and cooling optical part materials such as calciumfluoride, and to a single crystal production method using the crucible.

BACKGROUND ART

The Vertical Bridgman (VB) method is a known method for producing singlecrystals of calcium fluoride. The VB method involves moving a cruciblecarrying a calcium fluoride material vertically through a crystal growthfurnace which has a temperature gradient near the melting point of thecalcium fluoride material. In other words, first the crucible is raisedto melt the calcium fluoride material, and then it is gradually loweredin temperature (i.e. cooled) for gradual crystallization from bottom totop to grow a single crystal of the calcium fluoride. In some methods,the calcium fluoride material is melted by simple temperature control ofthe heater of the crystal growth furnace, without raising thetemperature of the crucible.

As an example of a crucible used for the VB method there may bementioned the crucible described in patent document 1. The crucibledisclosed in patent document 1 has a construction such that the calciumfluoride grows in a single crystal along the crystal plane of the seed(seed crystal), and it comprises a large-diameter starting materialcarrying section in which the calcium fluoride material is loaded, and asmall-diameter seed carrying section situated below the startingmaterial carrying section, wherein the calcium fluoride seed is loaded;the starting material carrying section and seed carrying section areconnected via a tapered cone surface.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The present inventors found that cooling of the calcium fluoride meltedin the aforementioned crucible tends to produce polycrystals or aheterophase, making it difficult to easily grow a single crystal ofcalcium fluoride.

It is therefore an object of the present invention to provide a cruciblewhich inhibits polycrystallization (heterophase formation) and aidssingle crystal growth, in order to facilitate growth of single crystalsof optical part materials such as calcium fluoride, as well as a methodfor growing single crystals.

As a result of much diligent research, the present inventors discoveredthat the problem described above occurs for the following reason.

Specifically, when the optical part material such as calcium fluoridewhich has melted in the crucible crystallizes along the crystal plane ofthe seed upon cooling, fine irregularities act as nuclei on the innersurface of the crucible and thus tend to cause production ofpolycrystals or a heterophase. When the optical part material contractsupon cooling, the optical part material such as calcium fluoride adheresto the inner surface of the crucible and creates residual stress orwarping in the crystals, tending to form the origins of crystalboundaries and thus contributing to the polycrystals or heterophase.

The present inventors therefore conducted diligent research with the aimof overcoming the problem described above, and discovered that theproblem can be overcome by the following invention.

Specifically, the crucible of the invention is a crucible for growth ofa single crystal along the crystal plane of a seed by melting andcooling an optical part material, characterized in that the surfaceroughness of the inner surface of the crucible as measured by themaximum height method is no greater than Rmax 6.4s. Throughout thepresent specification, the surface roughness Rmax is the value accordingto JIS B0601-1982.

Since the inner surface of the crucible of the invention is a smoothsurface with a surface roughness of no greater than Rmax 6.4s, theoptical part material which has melted in the crucible crystallizesalong the crystal plane of the seed upon cooling, thereby inhibitingformation of polycrystal-producing nuclei on the inner surface of thecrucible. In addition, since the optical part material easily separatesfrom the inner surface of the crucible as a result of contraction bycooling, residual stress or warping in the crystals of the optical partmaterial is inhibited. As a result, it is easy to grow a single crystalof the optical part material.

The surface roughness of the inner surface of the crucible of theinvention by the maximum height method is preferably no greater thanRmax 3.2s, and more preferably no greater than Rmax 2.0s.

If the inner surface of the crucible of the invention is composed ofglossy vitrified carbon, it will be easy to obtain a crucible innersurface with a surface roughness of no greater than Rmax 3.2s. In thiscase, the crucible body is preferably constructed of a carbon materialwhich has high heat resistance and readily produces a smooth surface.

The present inventors further discovered that the problem describedabove also occurs for the following reason.

Specifically, when the optical part material which has melted in thecrucible crystallizes along the crystal plane of the seed upon cooling,nuclei are formed by the corner of the boundary between the wall surfaceand the cone surface of the starting material carrying section of thecrucible inner surface and by the corner of the boundary between thecone surface and the wall surface of the seed carrying section, therebytending to cause formation of polycrystals and a heterophase. Also, whenthe optical part material contracts by cooling, the optical partmaterial adheres to the corners on the inner surface of the crucible,creating residual stress or warping in the crystals, which form theorigins of crystal boundaries and thus contribute to formation ofpolycrystals or a heterophase; as a result, a single crystal of theoptical part material does not readily grow.

The present inventors therefore conducted further diligent research andfound that this problem can be solved by the following invention.

Specifically, the crucible of the invention is a crucible for growth ofa single crystal along the crystal plane of a seed by melting andcooling an optical part material, characterized in that a tapered conesurface is formed between the starting material carrying section inwhich the optical part material is loaded and the seed carrying sectionin which the seed is loaded, the wall surface of the starting materialcarrying section is smoothly connected to the cone surface via a concavecurved plane, and the cone surface is smoothly connected to the wallsurface of the seed carrying section via a convex curved plane.

In the crucible of the invention, the wall surface of the startingmaterial carrying section is smoothly connected to the cone surface viaa concave curved plane and the cone surface is smoothly connected to thewall surface of the seed carrying section via a convex curved plane,such that when the optical part material which has melted in thecrucible crystallizes along the crystal plane of the seed upon cooling,generation of nuclei responsible for polycrystallization on the crucibleinner surface is inhibited. In addition, since the optical part materialeasily separates from the crucible inner surface as a result ofcontraction by cooling, residual stress or warping in the crystals ofthe optical part material is inhibited. As a result, it is easy to growa single crystal of the optical part material.

In the crucible of the invention, the curvature radius of the curvedplane connecting the wall surface of the starting material carryingsection of the crucible inner surface, the cone surface and the wallssurface of the seed carrying section is preferably at least 1/10, morepreferably at least ⅙ and most preferably at least ¼ of the innerdiameter between the wall surfaces of the starting material carryingsection.

The present inventors further discovered that the problem describedabove also occurs for the following reason.

Specifically, when the optical part material which has melted in thecrucible crystallizes along the crystal plane of the seed upon cooling,if the cone angle of the cone surface is too small, residual stress orwarping can result in the crystals, tending to form the origins ofcrystal boundaries and thus contributing to the polycrystals orheterophase. On the other hand, if the cone angle of the cone surface istoo large, growth of the single crystal will be inhibited, making itimpossible to easily achieve growth of a single crystal of the opticalpart material.

The present inventors therefore conducted further diligent research andfound that this problem can be solved by the following invention.

Specifically, the crucible of the invention is a crucible for growth ofa single crystal along the crystal plane of a seed by melting andcooling an optical part material, characterized in that the cone angleof the tapered cone surface formed between the starting materialcarrying section in which the optical part material is loaded and theseed carrying section in which the seed is loaded is set in a rangebetween 95° and 150°.

Since the cone angle of the cone surface in the crucible of theinvention is set to at least 95°, residual stress or warping in thecrystals which produces crystal boundaries is inhibited, and thuspolycrystallization (heterophase formation) is inhibited, when theoptical part material which has melted in the crucible crystallizesalong the crystal plane of the seed upon cooling. Also, since the coneangle of the cone surface is set to no greater than 150°, growth of asingle crystal is aided. As a result, single crystals of the opticalpart material can be easily grown.

In the crucible of the invention, the cone angle of the cone surface ispreferably set in a range between 105° and 140°, and more preferably ina range between 120° and 130°.

The present inventors further discovered that the problem describedabove also occurs for the following reason.

Specifically, when the optical part material melted in the cruciblecontracts by cooling, the optical part material adheres to the crucibleinner surface, creating residual stress or warping in the crystals andthus forming the origins of crystal boundaries and contributing toformation of polycrystals or a heterophase; as a result, a singlecrystal of the optical part material does not readily grow.

The present inventors therefore conducted further diligent researchtoward a solution to the problem. The following conclusion was reachedas a result of experimentation on the growth of single crystals ofoptical part materials in crucibles. Specifically, it was confirmed thata higher degree of wettability between the crucible inner surface andwater droplets permits more satisfactory single crystal growth. Inaddition, it was found that if the contact angle between the crucibleinner surface and water droplets is 100° or smaller, the wettabilitybetween the crucible inner surface and the optical part materialsolution is lower, thereby allowing satisfactory growth of a singlecrystal, and the present invention was thereupon completed.

In other words, the crucible of the invention is a crucible for growthof a single crystal along the crystal plane of a seed by melting andcooling an optical part material, characterized in that the contactangle between the crucible inner surface and water droplets is nogreater than 100°.

Since the contact angle between the crucible inner surface and waterdroplets in the crucible of the invention is 100° or smaller andwettability between the crucible inner surface and the optical partmaterial solution is low, the optical part material easily separatesfrom the crucible inner surface when the optical part material melted inthe crucible contracts by cooling, and therefore residual stress orwarping in the crystals of the optical part material is inhibited. As aresult, it is easy to grow a single crystal of the optical partmaterial.

In the crucible of the invention, the contact angle between the crucibleinner surface and water droplets in the crucible of the invention ispreferably 90° or smaller and more preferably 85° or smaller. Thecrucible inner surface is preferably composed of vitrified carbon, andthe sections other than the crucible inner surface are preferablycomposed of a carbon material.

The present inventors further discovered that the problem describedabove also occurs for the following reason.

Specifically, when a single crystal of an optical part material is grownusing the crucible described above, the crucible carrying the seed inthe seed carrying section and the optical part starting material in thestarting material carrying section is set in a crystal growth furnaceand a vacuum is created in the interior of the crystal growth furnacewhile forming a temperature gradient, to raise and lower the crucibleand produce melting and cooling of the starting material and a portionof the seed for single crystal growth. It is difficult to control thetemperature so that only a portion of the seed melts during melting ofthe optical part starting material, and the seed may melt entirely insome cases. Melting of the entirety of the seed makes it difficult toobtain a single crystal with the desired crystal orientation. In otherwords, the yield of desired single crystals is vastly reduced.

The crucible is therefore connected to a cooling rod via a support rod,and the crucible is cooled by the cooling rod through the support rodduring melting of the starting material, thereby cooling the bottom ofthe seed.

However, because the seed carrying section of the crucible used for thesingle crystal growth method described above is usually formed by apoint ended drill, the bottom face of the seed carrying section isconical. On the other hand, the bottom of the seed is generally flat.Consequently, when the seed is loaded in the seed carrying section, agap remains between the seed bottom face and the seed carrying sectionbottom face. As a result, the bottom of the seed is not sufficientlycooled during melting of the starting material, potentially resulting inmelting of the entirety of the seed and hampering efforts to obtain asingle crystal with the desired crystal orientation.

The present inventors therefore conducted further diligent research andfound that this problem can be solved by the following aspect of theinvention.

Specifically, the present invention provides a crucible for growth of asingle crystal along the crystal plane of an optical part material seedby melting and cooling an optical part material, characterized bycomprising a starting material carrying section in which the opticalpart starting material is loaded and a seed carrying section in whichthe seed is loaded, wherein the bottom of the seed carrying section hasa shape matching the edge of the seed.

Since the bottom of the seed carrying section has a shape matching theedge of the seed in this crucible, it is possible to adequately reducethe gap formed between the surface of the seed carrying section bottomand the seed edge surface when the seed is loaded in the seed carryingsection. As a result, when the optical part material is further loadedas the starting material in the starting material carrying section ofthe crucible and the starting material is melted while cooling thebottom of the seed through the crucible, the seed bottom is keptsufficiently cooled. Consequently, melting of the entirety of the seedcan be adequately prevented.

Specifically, in the crucible described above, the edge of the seed hasan end face and sides connected to the end face, while the bottom of theseed carrying section has a bottom face and a wall surface which isconnected to the bottom face and matches the sides of the seed, whereinboth the end face and the bottom face are flat surfaces.

The present invention further provides a single crystal growth methodwhereby a single crystal of an optical part material is grown using theaforementioned crucible, characterized by comprising a seed loading stepin which a seed having an edge with a shape matching the bottom of theseed carrying section is loaded as a seed in the seed carrying sectionof the crucible, a starting material loading step in which the opticalpart material is loaded as the starting material in the startingmaterial carrying section, and a growth step in which a single crystalof the optical part material is grown along the crystal planes of theseed by melting and cooling the starting material in the crucible.

According to this single crystal growth method, the bottom of the seedcarrying section has a shape matching the edge of the seed, andtherefore when the seed is loaded in the seed carrying section of thecrucible, the gap formed between the surface of the bottom of the seedcarrying section and the edge surface of the seed can be adequatelyminimized. As a result, when the optical part material is further loadedas the starting material in the starting material carrying section ofthe crucible and the starting material is melted while cooling thebottom of the seed through the crucible, the bottom of the seed is keptsufficiently cooled. Consequently, melting of the entirety of the seedcan be adequately prevented.

The present inventors further discovered that the problem describedabove also occurs for the following reason.

Specifically, by growing a single crystal in the crucible using theunmelted portion of the seed loaded in the seed carrying section as theseed, it is possible to achieve a uniform crystal orientation. If theentirety of the seed in the seed carrying section is melted it becomesimpossible to control the orientation. However, the desired resultcannot be achieved even if the seed portion near the top area of theseed carrying section has been melted. Experience has shown thatsubsequent single crystal growth is satisfactorily accomplished if theseed portion near the center area of the seed carrying section ismelted. It has therefore been the conventional practice to control thetemperature of the heater of the crystal growth furnace so that the seedmelts up to the center area of the seed carrying section.

With this single crystal growth method, however, it has been difficultto consistently melt the seed in the seed carrying section to thedesired point because of differences in the material components, anddepending on the degree of vacuum in the crystal growth furnace or thetemperature of the cooling water when cooling water is used to controlcooling of the crucible. As a result, repeated trial and error has beenindispensable for melting of a seed in the seed carrying section to thedesired point in order to obtain the desired single crystal.

The present inventors therefore conducted further diligent research andfound that this problem can be solved by the following invention.

Specifically, the present invention provides a crucible for growth of asingle crystal characterized by comprising a closed-bottom seed carryingsection extending in the vertical direction, in which the seed isloaded, a starting material carrying section in which a single crystalof the starting material is loaded, which is situated above the seedcarrying section and is connected to the seed carrying section, andtemperature detecting means for detection of the internal temperature ofthe seed carrying section. Here, the “vertical direction” is the uprightdirection.

The invention further provides a single crystal growth method for anoptical part material using the aforementioned crucible, which methodcomprises a step of preparing a crucible, a step of loading the seed ofthe optical part material in the seed carrying section, a step ofloading the starting material for the single crystal of the optical partmaterial in the starting material carrying section, a step of situatingthe crucible in a crystal growth furnace heated in such a manner thatthe interior has a specified temperature gradient in the verticaldirection, and heating the crucible so that the starting material loadedin the starting material carrying section and the seed loaded in theseed carrying material gradually melt from top to bottom, a step ofdetecting the internal temperature of the seed carrying section by thetemperature detecting means during heating of the crucible, and a stepof terminating the heating and commencing cooling for growth of a singlecrystal when, based on the internal temperature of the seed carryingsection detected by the temperature detecting means, the boundaryposition between the melted portion and unmelted portion of the seedloaded in the seed carrying section is judged to be between a firstposition which is at a prescribed height above the bottom end of theseed carrying section and a second position which is at a prescribedheight above the first position.

Thus, since the internal temperature of the seed carrying section can bedetected by the temperature detecting means, it is possible to easilydetermine the boundary position between the melted and unmelted sectionsof the seed loaded in the seed carrying section, based on the internaltemperature detected.

The temperature detecting means may be a thermocouple, and thethermocouple is preferably situated at a location near the side wall ofthe seed carrying section. This will facilitate measurement of thetemperature of the seed near the area in which the thermocouple issituated.

Also, a plurality of thermocouples are preferably situated at mutuallyseparated spacings in the vertical direction. This will allow thetemperature gradient of the seed carrying section to be known based onthe position of each thermocouple and the temperature measured from eachthermocouple.

Here, it is effective if one of the two thermocouples is situated at aposition at a height above the bottom end of the seed carrying sectioncorresponding to 25-50% of the depth of the seed carrying section, whilethe other is situated at a position at a height above the bottom end ofthe seed carrying section corresponding to 60-80% of the depth of theseed carrying section. Thus, if the seed melts within the range definedby the two thermocouple positions, satisfactory growth of a singlecrystal will occur thereafter. Such a state can be easily confirmedbased on whether the melting point of the seed falls between thetemperatures measured by the two thermocouples.

According to the optical part material single crystal growth method ofthe invention, it is effective if the first position is a position at aheight above the bottom end of the seed carrying section correspondingto 25% of the depth of the seed carrying section, and the secondposition is a position at a height above the bottom end of the seedcarrying section corresponding to 80% of the depth of the seed carryingsection. If heating is terminated and cooling initiated for singlecrystal growth when the boundary position between the melted andunmelted portions of the seed loaded in the seed carrying section fallsbetween the first and second positions, the single crystal of theoptical part material will grow satisfactorily along the crystal planeof the unmelted portion of the seed.

The crucible and single crystal growth method using it which aredescribed above are particularly effective when the optical partmaterial is calcium fluoride.

EFFECT OF THE INVENTION

If the surface roughness of the inner surface of the crucible is nogreater than Rmax 6.4s, i.e. if the surface is smooth, generation ofpolycrystal-forming nuclei on the crucible inner surface is inhibitedwhen the calcium fluoride melted in the crucible crystallizes along thecrystal surface of the seed by cooling. In addition, since the calciumfluoride easily separates from the crucible inner surface when itcontracts by cooling, residual stress and warping in the calciumfluoride crystals are inhibited. According to the invention, therefore,it is possible to easily grow single calcium fluoride crystals.

If the wall surface of the starting material carrying section issmoothly connected to the cone surface via a concave curved plane andthe cone surface is smoothly connected to the wall surface of the seedcarrying section via a convex curved plane, in the crucible of theinvention, generation of polycrystal-forming nuclei on the crucibleinner surface will be inhibited when the calcium fluoride melted in thecrucible crystallizes along the crystal surface of the seed by cooling.Also, since the calcium fluoride easily separates from the crucibleinner surface when it contracts by cooling, residual stress and warpingin the calcium fluoride crystals are inhibited. According to theinvention, therefore, it is possible to easily grow a single calciumfluoride crystal.

Furthermore, if the cone angle of the cone surface in the crucible ofthe invention is set to at least 95°, residual stress or warping in thecrystals which produces crystal boundaries will be inhibited, and thuspolycrystallization (heterophase formation) will be inhibited, when thecalcium fluoride which has melted in the crucible crystallizes along thecrystal plane of the seed upon cooling. Also, since the cone angle ofthe cone surface is set to no greater than 150°, growth of a singlecrystal will be aided. According to the invention, therefore, it ispossible to easily grow single calcium fluoride crystals.

Moreover, if the contact angle between the crucible inner surface andwater droplets is 100° or smaller and wettability between the crucibleinner surface and the calcium fluoride solution is low in the crucibleof the invention, the calcium fluoride will easily separate from thecrucible inner surface when the calcium fluoride melted in the cruciblecontracts by cooling, and therefore residual stress or warping in thecrystals of the calcium fluoride will be inhibited. According to theinvention, therefore, it is possible to easily grow single calciumfluoride crystals.

According to the crucible of the invention and the single crystal growthmethod employing the crucible of the invention, wherein the cruciblecomprises a starting material carrying section in which an optical partmaterial is loaded as the starting material and a seed carrying sectionin which the seed is loaded, if the bottom of the seed carrying sectionhas a shape matching the edge of the seed, the seed bottom can be keptsufficiently cooled during melting of the starting material whilecooling the seed bottom via the crucible, when the seed is loaded in theseed carrying section and the starting material is loaded in thestarting material carrying section. Consequently, melting of theentirety of the seed can be adequately prevented, and a single crystalcan be easily grown.

In addition, if the crucible is provided with the aforementionedtemperature detecting means, for the crucible of the invention and thesingle crystal growth method employing it, the boundary position betweenthe melted portion and unmelted portion of the seed loaded in the seedcarrying section can be easily determined, thereby allowing growth of asingle crystal of the calcium fluoride to be accomplished more reliably.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the general structure of a vacuumVB furnace provided with a crucible according to an embodiment of theinvention.

FIG. 2 is a cross-sectional view showing the structure of a crucibleaccording to the embodiment shown in FIG. 1.

FIG. 3 is a point graph showing the distribution of crystal orientationfor crystals obtained where the crucible shown in FIG. 1 was loweredinto the vacuum VB furnace 2 at an imperceptible speed and the growthrate was 1.0 mm/h.

FIG. 4 is a point graph showing the distribution of crystal orientationfor crystals obtained where the crucible shown in FIG. 1 was loweredinto the vacuum VB furnace 2 at an imperceptible speed and the growthrate was 2.0 mm/h.

FIG. 5 is a magnified view of the bottom of the seed carrying sectionaccording to a second embodiment of the crucible of the invention.

FIG. 6 is a schematic diagram showing the general structure of a vacuumVB furnace provided with a crucible according to a third embodiment ofthe invention.

FIG. 7 is a cross-sectional view showing the structure of a crucibleaccording to the third embodiment shown in FIG. 6.

FIG. 8 is a graph showing the relationship between crucible position,heater temperature and thermocouple temperature for a vacuum VB furnaceprovided with a crucible according to an experimental example of theinvention.

FIG. 9 is a table showing the experimental results of crystal state andyield for Experimental Examples 13-20 of the invention.

EXPLANATION OF SYMBOLS

-   1 Crucible-   1A Crucible body-   1B Cover member-   1C Bottom member-   1D Starting material carrying section-   1E Seed carrying section-   1F Cone surface-   1H Wall surface of starting material carrying section-   1J Concave curved plane-   1K Wall surface of seed carrying section-   1L Convex curved plane-   2 Vacuum VB furnace (Crystal growth apparatus furnace)-   2A Heater-   2B Shaft-   2C Vacuum pump-   2D Heat transfer member-   M Calcium fluoride (CaF₂) starting material-   S Calcium fluoride (CaF₂) seed (seed crystal)-   S1 Seed end face-   1CA, 1CB Thermocouples (temperature detecting means)-   100 Crystal growth apparatus

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the crucible of the invention will now be explained withreference to the accompanying drawings. Of the drawings, FIG. 1 is aschematic diagram showing the general structure of a vacuum VB furnaceprovided with a crucible according to an embodiment, and FIG. 2 is across-sectional view showing the structure of a crucible according tothe embodiment shown in FIG. 1. These embodiments will be explained withthe assumption that the seed (seed crystal) is composed of calciumfluoride.

First Embodiment

As shown in FIG. 1, the crucible 1 of this embodiment is situated on theinner side of a heater 2A in a vacuum Vertical Bridgman (hereinafterabbreviated as “VB”) furnace, as a single crystal growth apparatus forthe VB method. The crucible 1 is raised and lowered at an imperceptiblespeed via a shaft 2B to melt the calcium fluoride (CaF₂) startingmaterial M and then cooled to grow a single crystal along the crystalplane of, for example, the (111) orientation of a seed (seed crystal) Scomposed of a calcium fluoride (CaF₂) single crystal.

The pressure inside the vacuum VB furnace 2 is reduced to below 10⁻⁴ Pausing a vacuum pump 2C, and the heater 2A is used for heating to around,for example, 1400-1500° C. In order to prevent melting of the seed S byheating with the heater 2A, the shaft 2B of the vacuum VB furnace 2comprises a cooling water circulation channel.

That is, the shaft 2B is composed of a double conduit wherein the upperend of the inner conduit 2B1 is retired from the upper end of the outerconduit 2B2, with a cap-like heat transfer member 2D fitted at the upperend. Also, the heat transfer member 2D is connected to the center of thebottom member 1C of the crucible 1 described hereunder, for aconstruction wherein the lower part of the seed S is forcefully cooled.

As shown in FIG. 2, the crucible 1 comprises a crucible body 1A, a capmember 1B which covers the opening of the crucible body 1A, and a bottommember 1C anchored to the lower part of the crucible body 1B. Thecrucible body 1A is heat resistant, and is made of a high-purity carbonmaterial (C), as a material which increases the smoothness of the innerwall, while the inner wall is coated with glass-like carbon (GC) whichis glossy and capable of lowering the wettability with the calciumfluoride (CaF₂) solution.

A large-diameter starting material carrying section 1D, in which thecalcium fluoride (CaF₂) starting material M (FIG. 1) is loaded, isformed in the crucible body 1A. The starting material carrying section1D comprises a cylindrical wall surface 1H, a concave curved plane 1Jformed in connection with the bottom member 1C end of the wall surface1H, and a tapered (funnel-shaped) cone surface IF formed in connectionwith the bottom member 1C end of the concave curved plane 1J. Thus, thecone surface 1F forms the bottom of the starting material carryingsection 1D. A small seed carrying section 1E in which the cylindricalseed S (see FIG. 1) is loaded is formed as a straight circular hole atthe center section from the crucible body 1A, through the bottom member1C. The tapered (funnel-shaped) cone surface 1F forming the bottom ofthe starting material carrying section 1D is formed between the startingmaterial carrying section 1D and the seed carrying section 1E.

The cap member 1B and bottom member 1C are also formed of aheat-resistant high purity carbon material. A connecting cylindricalsection 1C1 is formed in a protruding manner at the center of the lowerside of the bottom member 1C, for fitting of the heat transfer member 2D(shown in FIG. 1) which is anchored to the top end of the shaft 2B ofthe vacuum VB furnace 2.

A concave curved plane 1J is formed at the border section between thewall surface 1H and cone surface 1F of the starting material carryingsection 1D, and the wall surface 1H and cone surface 1F of the startingmaterial carrying section 1D are in a smooth connection via the concavecurved plane 1J. Also, a convex curved plane 1L is formed at the bordersection between the cone surface 1F and the wall surface 1K of the seedcarrying section 1E, and the cone surface 1F and the wall surface 1K ofthe seed carrying section 1E are in a smooth connection via the convexcurved plane 1L.

The wall surface 1H of the starting material carrying section 1D isformed straight, and the inner diameter within the wall surface 1H maybe set to, for example, 250 mm. The inner diameter of the seed carryingsection 1E may be set to, for example, 20 mm.

If the cone angle θ of the cone surface 1F is too small, residual stressand warping will occur within the calcium fluoride (CaF₂) crystals grownin the starting material carrying section 1D, tending to result inpolycrystallization or heterophase formation. On the other hand, if thecone angle θ of the cone surface 1F is too large, growth of the calciumfluoride (CaF₂) single crystals will tend to be inhibited. Therefore,the cone angle θ of the cone surface 1F is preferably set to within95-150°, and most preferably 120-130°.

If the concave curved plane 1J and convex curved plane 1L have smallcurvature radii approaching angular forms, the angular concave curvedplane 1J and convex curved plane 1L sections will tend to act as nucleito produce polycrystals and a heterophase when the calcium fluoride(CaF₂) melted in the starting material carrying section 1D crystallizesby cooling. In addition, when the calcium fluoride (CaF₂) contracts uponcooling, the calcium fluoride (CaF₂) will adhere to the angular concavecurved plane 1J and convex curved plane 1L to generate residual stressand warping inside the crystals, tending to produce polycrystals and aheterophase.

The curvature radii of the concave curved plane 1J and convex curvedplane 1L are therefore set to be large curvature radii of at least 1/10of the inner diameter within the wall surface 1H of the startingmaterial carrying section 1D (for example, 250 mm). For example, thecurvature radius of the concave curved plane 1J may be set to about 60mm, and the curvature radius of the convex curved plane 1L may be set toabout 50 mm.

Also, if surface roughness of the wall surface 1H or cone surface 1F ofthe starting material carrying section 1D is high, the minuteirregularities in the wall surface 1H or cone surface 1F will tend toform polycrystals and a heterophase when the calcium fluoride (CaF₂)melted in the starting material carrying section 1D crystallizes uponcooling. In addition, shrinkage of the calcium fluoride (CaF₂) uponcooling will result in adhesion of the calcium fluoride (CaF₂) to thewall surface 1H or cone surface 1F to produce residual stress or warpinginside the crystal, tending to produce polycrystals or a heterophase.

The inner wall of the crucible 1 extending from the wall surface 1H ofthe starting material carrying section 1D of the crucible body 1A,through the concave curved plane 1J, cone surface 1F and convex curvedplane 1L, through to the wall surface 1K of the seed carrying section1E, is finished to a surface roughness of no greater than Rmax 6.4s,such as no greater than 3.2s, according to the maximum height method.That is, the inner surface of the crucible body 1A composed of the highpurity carbon material (C) is finished to, for example, about Rmax 6.4s,and the surface thereof is coated with glass-like carbon (GC) andfinished to about Rmax 3.2s.

If the wettability between the crucible inner surface, such as the wallsurface 1H or cone surface 1F of the starting material carrying section1D, and the calcium fluoride (CaF₂) solution is high, shrinkage of thecalcium fluoride (CaF₂) melted in the starting material carrying section1D upon cooling will result in adhesion of the calcium fluoride (CaF₂)to the wall surface 1H or cone surface 1F to produce residual stress orwarping inside the crystals, tending to produce polycrystals or aheterophase.

Thus, since the crucible inner surface composed of glass-like carbon(GC) having a surface roughness of no greater than Rmax 3.2s, i.e. thecrucible inner wall extending from the wall surface 1H of the startingmaterial carrying section 1D of the crucible body 1A, through theconcave curved plane 1J, cone surface 1F and convex curved plane 1L,through to the wall surface 1K of the seed carrying section 1E, has lowwettability with the calcium fluoride (CaF₂) solution, the water dropletcontact angle is no greater than 100°, and for example, 90°.

In order to melt the calcium fluoride (CaF₂) starting material M shownin FIG. 1 in the crucible 1 having this construction, the inner surfaceof the heater 2A heated to about 1400-1500° C. is raised at animperceptible speed of about 10 mm/h by the shaft 2B in the vacuum VBfurnace 2 (see FIG. 1) at a reduced pressure of no greater than 10⁻⁴ Pa,and is held at that raised position for about 10 hours. During thatperiod, the lower part of the seed S is forcefully cooled through theheat transfer member 2D by cooling water circulating in the shaft 2Bfrom the inner conduit 2B1 to the outer conduit 2B2, thereby preventingmelting of the sections other than the top of the seed S.

In order to cool the melted calcium fluoride (CaF₂) starting material Mto grow a single crystal along the crystal plane of, for example, the(111) orientation of the seed (seed crystal) S, the crucible 1 islowered by the shaft 2B at an imperceptible speed of no greater than 1.5mm/h, such as about 1.0 mm/h for example, and is held at the loweredposition in the vacuum VB furnace 2 for about 5 hours.

Then, the solidified calcium fluoride (CaF₂) in the crucible 1 is cooledat a cooling rate of no greater than 70° C./h, such as about 30° C./hfor example, by ON/OFF control of the heater 2A of the vacuum VB furnace2, in order to prevent quenching (cracking due to thermal shock).

In the crucible 1, the crucible inner wall extending from the wallsurface 1H of the starting material carrying section 1D of the cruciblebody 1A, through the concave curved plane 1J, cone surface 1F and convexcurved plane 1L, through to the wall surface 1K of the seed carryingsection 1E, is finished to a smooth surface of, for example, about Rmax3.2s. This inhibits generation of polycrystal-forming nuclei in thecrucible when the melted calcium fluoride (CaF₂) in the startingmaterial carrying section 1D cools and crystallizes along the crystalplane of the (111) orientation of the seed S.

Since the calcium fluoride (CaF₂) easily separates from the crucibleinner surface upon contraction by cooling, generation of residual stressor warping inside the calcium fluoride (CaF₂) crystals is inhibited.Growth of the calcium fluoride (CaF₂) crystals is thus facilitated.

In the crucible 1, the wall surface 1H of the starting material carryingsection 1D of the crucible body 1A is smoothly connected to the conesurface 1F via the concave curved plane 1J having a large curvatureradius of about 60 mm, while the cone surface 1F is smoothly connectedto the wall surface 1K of the seed carrying section 1E via the convexcurved plane 1L having a large curvature radius of about 50 mm. In otherwords, the crucible inner surface which extends from the wall surface 1Hof the starting material carrying section 1D, through the concave curvedplane 1J, cone surface 1F and convex curved plane 1L, through to thewall surface 1K of the seed carrying section 1E, is smoothly continuouswithout any angularity.

Thus, generation of polycrystal-forming nuclei on the crucible innersurface is inhibited when the calcium fluoride (CaF₂) melted in thestarting material carrying section 1D crystallizes along the crystalplane of the (111) orientation of the seed S upon cooling. Also, sincethe calcium fluoride (CaF₂) easily separates from the crucible innersurface upon contraction by cooling, generation of residual stress orwarping inside the calcium fluoride (CaF₂) crystals is inhibited. Growthof the calcium fluoride (CaF₂) crystals thus occurs in a reliablemanner.

The cone angle θ of the cone surface 1F of the crucible body 1A of thecrucible 1 is set to at least 120° within the range of 95-150°, so thatit is not too small or too large. This will inhibit residual stress orwarping inside the crystals, and thus crystal boundary formation, whenthe calcium fluoride (CaF₂) melted in the starting material carryingsection 1D crystallizes along the crystal plane of the (111) orientationof the seed S upon cooling, thus inhibiting polycrystallization(heterophase formation). The cone angle θ is also set to no greater than130°. This will aid growth of a single crystal when the calcium fluoride(CaF₂) crystallizes along the crystal plane of the (111) orientation ofthe seed S upon cooling. Growth of the calcium fluoride single crystalcan thereby be reliably achieved.

The inner wall of the crucible body 1 coated with glass-like carbon(GC), i.e., the crucible inner wall extending from the wall surface 1Hof the starting material carrying section 1D of the crucible body 1A,through the concave curved plane 1J, cone surface 1F and convex curvedplane 1L, through to the wall surface 1K of the seed carrying section 1Ein the crucible 1, is finished to a water droplet contact angle of, forexample, 90° to minimally reduce the wettability between the crucibleinner surface and the calcium fluoride (CaF₂) solution.

Consequently, the calcium fluoride (CaF₂) melted in the crucible easilyseparates from the crucible inner surface upon contraction of thecalcium fluoride (CaF₂) by cooling. As a result, generation of residualstress or warping inside the calcium fluoride (CaF₂) crystal isinhibited, so that growth of the calcium fluoride (CaF₂) single crystalis facilitated.

The inner surface of the crucible body 1 is also finished to a smoothsurface of about Rmax 3.2s. Consequently, generation ofpolycrystal-forming nuclei on the crucible inner surface is inhibitedwhen the calcium fluoride (CaF₂) melted in the starting materialcarrying section 1D crystallizes along the crystal surface of the (111)orientation of the seed S by cooling. As a result, growth of the calciumfluoride (CaF₂) single crystal is facilitated.

In addition, since the calcium fluoride (CaF₂) solidified in thecrucible 1 is cooled at a cooling rate of no greater than 70° C./h, suchas about 30° C./h, for example, quenching (cracking due to thermalshock) is prevented and a satisfactory single crystal is grown.

Furthermore, since the imperceptible speed at which the crucible 1 islowered for cooling of the melted calcium fluoride (CaF₂) to grow asingle crystal, i.e. the growth rate, is no greater than 1.5 mm/h, suchas about 1.0 mm/h, for example, the crystal orientation of the grownsingle crystal is stable, as shown in FIG. 3. When the growth rate is 2mm/h, which is above 1.5 mm/h, the crystal orientation clearly becomesdispersed and unstable as shown in FIG. 4.

Second Embodiment

A second embodiment of the crucible of the invention will now beexplained. According to this embodiment, the crucible is suited forusing a seed having a cylindrical shape and a flat edge. FIG. 5 is amagnified view of the bottom of the seed carrying section.

The crucible 1 of this embodiment has a construction similar to thefirst embodiment, except that the seed carrying section 1E has a shapematching the seed S. More specifically, the seed carrying section 1E hasa construction wherein the bottom of the seed carrying section 1E isshaped to match the edge of the seed S.

The bottom of the seed carrying section 1E has such a shape matching theedge of the seed S in order to adequately minimize gaps between the sideforming the bottom of the seed carrying section 1E and the edge surfacesof the seed S, when the seed S is loaded in the seed carrying section1E.

More specifically, as shown in FIG. 5, the edge surface of the seed Shas a flat end face S1 and a cylindrical side S2 vertical to the endface S1 and continuous with the end face S1, while the side forming thebottom of the seed carrying section 1E has a flat bottom face 1N and acylindrical wall surface 1K vertical to the bottom face 1N andcontinuous with the bottom face 1N. The diameter within the wall surface1K roughly matches the diameter of the seed S. The wall surface 1K ofthe seed carrying section is smaller than that of the wall surface 1H ofthe starting material carrying section 1D.

A method of growing a calcium fluoride single crystal as an optical partmaterial using the crucible 1 described above will now be explained.

First, the crucible 1 is prepared, the cover member 1B is removed, andthe calcium fluoride seed S is loaded into the seed carrying section 1Eof the crucible 1 (seed loading step). The seed S has a cylindricalshape with a flat edge, and the diameter roughly matches the diameter ofthe wall surface 1K of the seed carrying section 1E. When the seed S isloaded into the seed carrying section 1E, at least the bottom of theseed carrying section 1E must conform to the edge of the seed S. Thus,it is possible to adequately reduce gaps between the side S2 of the seedS and the wall surface 1K of the seed carrying section 1E, and betweenthe end face S1 of the seed S and the bottom face 1N of the seedcarrying section 1E.

After the seed S has been loaded into the seed carrying section 1E, thecalcium fluoride starting material M is loaded into the startingmaterial carrying section 1D (starting material loading step).

The starting material carrying section 1D of the crucible body 1A isthen closed with the cover member 1B.

Next, the pressure inside the vacuum VB furnace 2 is reduced to below10⁻⁴ Pa, and the heater 2A is heated to about 1400-1500° C. Also, thecrucible 1 is raised at an imperceptible speed of about 10 mm/h by theshaft 2B and is held at the raised position for about 10 hours. Duringthat period, since it is difficult to obtain a single crystal with thedesired crystal orientation if the entire seed S is melted, the lowerpart of the seed S is forcefully cooled through the heat transfer member2D by cooling water circulating in the shaft 2B from the inner conduit2B1 to the outer conduit 2B2. If gaps form between the side S2 of theseed S and the wall surface 1K of the seed carrying section 1E, andbetween the end face S1 of the seed S and the bottom face 1N of the seedcarrying section 1E during this time, the lower heat conductivity of thegaps compared to the carbon of the bottom member 1C will preventadequately cooling of the bottom of the seed carrying section 1E;however, as mentioned above, gaps between the side S2 of the seed S andthe wall surface 1K of the seed carrying section 1E, and between the endface S1 of the seed S and the bottom face 1N of the seed carryingsection 1E, are adequately minimized in the crucible 1. Consequently,the bottom section of the seed S is sufficiently cooled so that meltingof the entirety of the seed S can be satisfactorily prevented.

After the calcium fluoride starting material M has melted, the crucible1 is lowered by the shaft 2B at an imperceptible speed of no greaterthan 1.5 mm/h, such as about 1.0 mm/h for example, and is held at thelowered position in the vacuum VB furnace 2 for about 5 hours. Thus, themelted calcium fluoride (CaF₂) starting material M is cooled to grow asingle crystal along the crystal plane of the (111) orientation, forexample, of the seed S (growth step).

Next, the solidified calcium fluoride (CaF₂) in the crucible 1 is cooledat a cooling rate of no greater than 70° C./h, such as about 30° C./hfor example, by ON/OFF control of the heater 2A of the vacuum VB furnace2, in order to prevent quenching (cracking due to thermal shock).

In the crucible 1, the crucible inner wall extending from the wallsurface 1H of the starting material carrying section 1D of the cruciblebody 1A, through the concave curved plane 1J, cone surface 1F and convexcurved plane 1L, through to the wall surface 1K of the seed carryingsection 1E, is finished to a smooth surface of, for example, about Rmax3.2s. This inhibits generation of polycrystal-forming nuclei on thecrucible inner surface when the melted calcium fluoride (CaF₂) in thestarting material carrying section 1D cools and crystallizes along thecrystal plane of the (111) orientation of the seed S.

Since the calcium fluoride (CaF₂) easily separates from the crucibleinner surface upon contraction by cooling, generation of residual stressor warping inside the calcium fluoride (CaF₂) crystals is inhibited. Asa result, growth of the calcium fluoride (CaF₂) crystals is facilitated.

In addition, since the calcium fluoride (CaF₂) solidified in thecrucible I is cooled at a cooling rate of no greater than 70° C./h, suchas about 30° C./h, for example, quenching (cracking due to thermalshock) is prevented and a satisfactory single crystal is grown.

Furthermore, since the crucible 1 is lowered at an imperceptible speedfor cooling of the melted calcium fluoride (CaF₂) to grow a singlecrystal, i.e. the growth rate is no greater than 1.5 mm/h, such as about1.0 mm/h, for example, the crystal orientation of the grown singlecrystal is stable, as shown in FIG. 4. When the growth rate is 2 mm/h,which is above 1.5 mm/h, the crystal orientation clearly becomesdispersed and unstable, as shown in FIG. 5.

According to this embodiment, the wall surface 1K of the seed carryingsection 1E of the crucible 1 is cylindrical, but the shape of the wallsurface 1K may be prismatic if a prismatic seed is to be loaded.

Moreover, the bottom face of the seed carrying section 1E is flat inthis case, but the bottom face of the seed carrying section is notlimited to being a flat surface. If the end face S1 of the seed S isconical, the bottom face 1N of the seed carrying section 1E may beconical. That is, the bottom face 1N of the seed carrying section 1E mayhave any shape which can satisfactorily reduce gaps between the end faceS1 of the seed S and the bottom face 1N of the seed carrying section 1Ewhen the seed S is loaded into the seed carrying section 1E.

Third Embodiment

A third embodiment of the crucible of the invention will now beexplained. As the drawings for reference, FIG. 6 is a schematic diagramshowing the general structure of a crystal growth apparatus providedwith a crucible according to this embodiment, and FIG. 7 is across-sectional view showing the structure of a crucible according tothe embodiment shown in FIG. 6.

As shown in FIG. 6, the crucible 1 of this embodiment is situated on theinner side of a primary heater 2AA and a secondary heater 2AB in avacuum VB furnace 2, as a single crystal growth apparatus 100 for the VBmethod. The crucible 1 is raised at an imperceptible speed via a shaft2B to melt the calcium fluoride starting material M and then lowered atan imperceptible speed via the shaft 2B to cool the starting material M,in order to grow a single crystal along the crystal plane of, forexample, the (111) orientation of the seed S composed of a calciumfluoride single crystal.

The pressure inside the vacuum VB furnace 2 is reduced to below 10 ⁻⁴ Pausing a vacuum pump 2C, and the upper portion of the vacuum VB furnace 2is heated to, for example, about 1600° C. by the primary heater 2AA. Thebottom portion of the vacuum VB furnace 2 is heated by the secondaryheater 2AB. Cooling water channels 2D, 2E are formed through a waterjacket 1G described hereunder, in the shaft 2B of the vacuum VB furnace2.

As shown in FIG. 7, the crucible 1 of this embodiment has a constructionprovided with a crucible body 1A and a cover member 1B covering theopening of the crucible body 1A. The crucible body 1A is heat resistant,and is made of a material, such as a high-purity carbon material, whichincreases the smoothness of the inner wall and lowers the wettability ofthe calcium fluoride starting material M. The cover member 1B is alsomade of a heat resistant high-purity carbon material.

A large-diameter starting material carrying section 1D is also formed inthe crucible body 1A, for loading of the calcium fluoride startingmaterial M (see FIG. 6). A smaller seed carrying section 1E in which,for example, a cylindrical seed S (see FIG. 6) is loaded is formed as astraight circular hole from the center section of the crucible body 1A,up to the area near the bottom. A cone surface 1F forming the bottom ofthe starting material carrying section 1D is formed between the startingmaterial carrying section 1D and the seed carrying section 1E. Theborder surface between the starting material carrying section 1D and theseed carrying section 1E is the upper edge of the seed S inserted intothe seed carrying section 1E, and it is determined by the length of theseed S.

In order to forcefully cool the lower part of the seed S loaded in theseed carrying section 1E (see FIG. 6) to prevent its melting, a waterjacket 1G is formed around the bottom of the seed carrying section 1E,at the bottom section of the crucible body 1A. The water jacket 1G formsa cooling water circulation channel in connection with the cooling waterchannels 2D, 2E in the shaft 2B (see FIG. 6).

A concave curved plane 1J is formed at the border section between thewall surface 1H and cone surface 1F of the starting material carryingsection 1D, and the wall surface 1H and cone surface 1F of the startingmaterial carrying section 1D are in a smooth connection via the concavecurved plane 1J. Also, a convex curved plane 1L is formed at the bordersection between the cone surface 1F and the wall surface 1K of the seedcarrying section 1E, and the cone surface 1F and the wall surface 1K ofthe seed carrying section 1E are in a smooth connection via the convexcurved plane 1L.

The wall surface 1H of the starting material carrying section 1D isformed straight, and the inner diameter within the wall surface 1H maybe set to, for example, 250 mm. The inner diameter of the seed carryingsection 1E may be set to, for example, 20 mm.

In the crucible 1 of this embodiment, a thermocouple 1CA andthermocouple 1CB are situated near the side wall of the seed carryingsection 1E, mutually separated in the vertical direction, as temperaturedetecting means for measurement of the internal temperature of the seedcarrying section. The vertical direction may be considered to be thedirection in which the seed carrying section 1E extends. In this case,the high purity carbon material used as the material for the cruciblebody 1A can be easily worked because it has a porous structure, andthermocouple insertion holes can be suitably formed to circumvent theeffect of the temperature in the crucible. The high purity carbonmaterial also does not react with metals normally used forthermocouples, and therefore the thermocouples can be satisfactorilysituated inside the crucible 1.

Here, thermocouples employing a platinum-rhodium alloy are preferablyused as the thermocouples 1CA, 1CB. Such thermocouples are optimal forprecision measurement at high temperature, and therefore allowmeasurement of the internal temperature of the seed carrying section 1E.As an example of a thermocouple employing platinum-rhodium there may bementioned JIS-Type B (+wire: platinum-rhodium alloy containing 30%rhodium, −wire: platinum-rhodium alloy containing 6% rhodium).

The two thermocouples allow a temperature gradient to be derived for theseed carrying section 1E based on the positions P1, P2 at which thethermocouple 1CA and thermocouple 1CB are situated and on thetemperatures measured from the thermocouple 1CA and thermocouple 1CB,and by calculating the position representing the melting point of theseed S it is possible to easily determine the boundary position betweenthe melted portion and the unmelted portion of the seed S in the seedcarrying section 1E.

The thermocouple 1CA is preferably situated at a position (firstposition) P1 at a height above position P0, which is the height of thebottom face 1N of the seed carrying section 1E, corresponding to 25-50%of the depth of the seed carrying section 1E, while the thermocouple 1CBis preferably situated at a position (second position) P2 at a heightabove position P0, which is the height of the bottom face 1N of the seedcarrying section 1E, corresponding to 60-80% of the depth of the seedcarrying section 1E. When the seed has melted up to the range defined bythe two points of the seed carrying section 1E whose temperatures havebeen measured by the thermocouple 1CA and thermocouple 1CB, subsequentsingle crystal growth will proceed in a satisfactory manner. Such astate may be easily judged by whether the melting point of the seed Sfalls between the temperatures measured by the thermocouple 1CA andthermocouple 1CB, without deriving the temperature gradient of the seedcarrying section 1E.

A method of growing a calcium fluoride single crystal using the crucible1 described above will now be explained.

First, the crucible 1 is prepared, the cover member 1B is removed, andthe calcium fluoride seed S is loaded into the seed carrying section 1Eof the crucible 1. Next, the calcium fluoride starting material M isloaded into the starting material carrying section 1D, and the startingmaterial carrying section 1D of the crucible body 1A is closed with thecover member 1B. The crucible 1 is in a lowered state by the shaft 2B atthis time.

The pressure inside the vacuum VB furnace 2 is reduced to below 10⁻⁴ Pa,and the primary heater 2AA is heated to about 1600° C. Also, thecrucible 1 is raised by the shaft 2B at an imperceptible speed of about10 mm/h and held at the raised position for about 10 hours.

At this time, the internal temperature of the seed carrying section 1Eis measured by the thermocouple 1CA and thermocouple 1CB, and moving ofthe crucible 1 is terminated when the boundary position between themelted portion and the unmelted portion of the seed S is judged to bewithin the zone of the prescribed heights from the bottom face 1N of theseed carrying section 1E, based on the internal temperatures. The zoneof the prescribed heights is preferably the range defined between thefirst position P1 at a height above the bottom face 1N of the seedcarrying section 1E corresponding to 25% of the depth of the seedcarrying section 1E and the second position P2 at a height above thebottom face 1N of the seed carrying section 1E corresponding to 80% ofthe depth of the seed carrying section 1E. More preferably, the firstposition P1 is at a height above the bottom face 1N of the seed carryingsection 1E corresponding to 40% of the depth of the seed carryingsection 1E while the second position P2 is at a height above the bottomface 1N of the seed carrying section 1E corresponding to 70% of thedepth of the seed carrying section 1E, and even more preferably, thefirst position P1 is at a height above the bottom face 1N of the seedcarrying section 1E corresponding to 50% of the depth of the seedcarrying section 1E while the second position P2 is at a height abovethe bottom face 1N of the seed carrying section 1E corresponding to 60%of the depth of the seed carrying section 1E. This will produce acondition for satisfactory growth of a calcium fluoride single crystalalong the crystal plane of the unmelted portion of the seed.

After moving of the crucible 1 has been terminated, the crucible 1 islowered by the shaft 2B at an imperceptible speed of about 1.0 mm/h, andheld at the lowered position in the vacuum VB furnace 2 for about 5hours. This allows cooling of the melted calcium fluoride startingmaterial M to grow a single crystal along the crystal plane of, forexample, the (111) orientation of the seed S, in order to obtain thedesired calcium fluoride single crystal.

This completes the explanation of the preferred embodiments of theinvention, but it is needless to mention that the invention is notlimited to these three embodiments.

Also, although the crucible 1 of the first embodiment satisfies all ofthe conditions (i) to (vi) listed below, a calcium fluoride singlecrystal can be easily grown so long as the crucible 1 satisfies any oneof the following conditions (i) to (vi).

-   (i) The crucible inner surface has a surface roughness of no greater    than Rmax 6.4s according to the maximum height method;-   (ii) A tapered cone surface is formed between the starting material    carrying section in which the calcium fluoride starting material is    loaded and the seed carrying section in which the seed is loaded,    and the wall surface of the starting material carrying section is    smoothly connected to the cone surface via a concave curved plane    while the cone surface is smoothly connected to the wall surface of    the seed carrying section via a convex curved plane;-   (iv) the cone angle of the tapered cone surface formed between the    starting material carrying section in which the calcium fluoride    starting material is loaded and the seed carrying section in which    the seed is loaded is set in a range between 95° and 150°;-   (v) the water droplet contact angle with the crucible inner surface    is no greater than 100°;-   (vi) the crucible comprises a starting material carrying section in    which the calcium fluoride starting material is loaded and a seed    carrying section in which the seed is loaded, and the bottom of the    seed carrying section has a shape matching the edge of the seed.

For example, raising the crucible 1 by the shaft 2B at an imperceptiblespeed in the third embodiment described above results in melting of thecalcium fluoride starting material M, but the same effect of the thirdembodiment may also be achieved by controlling the primary heater 2AA toheat the crucible 1 while the crucible 1 is already raised with theshaft 2B.

Also, a thermocouple may be situated at a position near the side wall ofthe starting material carrying section 1D of the third embodiment formeasurement of the internal temperature of the starting materialcarrying section 1D, thereby facilitating temperature regulation of theprimary heater 2AA and secondary heater 2AB. In addition, a thermometermay be situated in the water jacket 1G to facilitate temperatureregulation of the cooling water, in order to maintain a more constanttemperature for cooling of the lower portion of the seed S.

Also if the border surface between the starting material carryingsection 1D and seed carrying section 1E is below the convex curved plane1L in the third embodiment, calculation of the temperature gradient ofthe seed carrying section 1E can be accomplished even if thermocouplesare situated at a position near the surface defined between the convexcurved plane 1L and the wall surface 1K of the seed carrying section 1Eand at a position near the wall surface 1K of the seed carrying section1E.

In addition, although two thermocouples were situated in the crucible 1of the third embodiment, a single thermocouple may be situated at aposition near the side wall of the seed carrying section 1E in the zonedefined between a first position at a height above the bottom end of theseed carrying section 1E corresponding to 25% of the depth of the seedcarrying section 1E and a second position at a height above the bottomend of the seed carrying section 1E corresponding to 80% of the depth ofthe seed carrying section 1E. In this case, once the temperature of thethermocouple reaches the melting point of the seed S, the boundaryposition between the melted portion and unmelted portion of the seed Sin the seed carrying section 1E will have reached the position of thethermometer of the thermocouple, and therefore terminating moving of thecrucible 1 at this point will allow satisfactory growth of the calciumfluoride single crystal along the crystal plane of the unmelted portionof the seed. The preferred positions and particularly preferredpositions for the first position and second position in this case arethe same as for the third embodiment described above.

In the first to third embodiments described above the seed was composedof calcium fluoride, but the crucible of the invention and the singlecrystal growth method employing it may also be applied for optical partmaterials wherein the seed is a material other than calcium fluoride(for example, barium fluoride or magnesium fluoride).

EXPERIMENTAL EXAMPLES

Experimental examples will now be described.

Experimental Examples 1-5

For Experimental Examples 1-5, crucibles 1 having different innersurface roughnesses of the crucible body 1A and different glass-likecarbon (GC) coating thicknesses were used for growth of calcium fluoride(CaF₂) single crystals with a vacuum VB furnace 2, and the incidence ofpolycrystal formation in the obtained crystals was measured andevaluated.

The surface roughness was measured according to the maximum heightmethod, by three-dimensional shape measurement using an OLS1100 scanningconfocal laser microscope by Shimadzu Laboratories. The incidence ofpolycrystals was observed using two Polar films by Edmund IndustrialOptics (color: gray, area: 15 in.×8.5 in., thickness: 0.29 mm).Specifically, the two films were placed together with their film sidesfacing parallel and sandwiching the crystal, and the crystal wasobserved from one side while irradiating a light source from theopposite side of the film. The crystal angle and position were changedand the non-single crystal portion was measured as polycrystalline. Thefinal polycrystalline portion volume was calculated, and the ratio ofthe total crystal volume and the polycrystalline portion volume wasdetermined as the incidence of polycrystal formation. Crystals with anincidence of polycrystal formation of less than 30% were evaluated as ⊚,and those above 70% were evaluated as ∘.

The evaluation results are shown in Table 1, which clearly shows that ifthe crucible inner surface is coated with glass-like carbon (GC) and hasa surface roughness of about Rmax 3.2s, as in Experimental Example 1 andExperimental Example 2, the incidence of polycrystal formation is 30% orlower and a calcium fluoride (CaF₂) single crystal is easily grown. Italso clearly shows that even if no glass-like carbon (GC) coating ispresent and the surface roughness of the crucible inner surface is aboutRmax 6.4s, as in Experimental Example 3, the incidence of polycrystalformation is still 30% or lower and a calcium fluoride (CaF₂) singlecrystal is easily grown.

However, in the case of a rough surface where the surface roughness ofthe crucible inner surface is below Rmax 25s, as in ExperimentalExamples 4 and 5, regardless of whether or not a glass-like carbon (GC)coating is present, the incidence of polycrystal formation is 70% orgreater. This demonstrated that the crucibles of Experimental Examples1-3 allowed easier growth of calcium fluoride (CaF₂) single crystalscompared to the crucibles of Experimental Examples 4-5. TABLE 1 Crucibleinner surface GC coating roughness thickness Evaluation Exp. Example 1R_(max) 3.2s 2.0 mm ⊚ Exp. Example 2 R_(max) 3.2s 1.0 mm ⊚ Exp. Example3 R_(max) 6.4s   0 mm ⊚ Exp. Example 4 R_(max) 25s 1.0 mm ∘ Exp. Example5 R_(max) 50s   0 mm ∘

Experimental Examples 6-10

For Experimental Examples 6-10, crucibles 1 having different contactangles between the inner surface of the crucible body 1A and waterdroplets, and different glass-like carbon (GC) coating thicknesses wereused for growth of calcium fluoride (CaF₂) single crystals with a vacuumVB furnace 2, and the incidence of polycrystal formation in the obtainedcrystals was measured and evaluated.

The contact angle was measured by the maximum height method, bythree-dimensional shape measurement using an OLS1100 scanning confocallaser microscope by Shimadzu Laboratories. The incidence of polycrystalswas observed using two Polar films by Edmund Industrial Optics (color:gray, area: 15 in.×8.5 in., thickness: 0.29 mm). Specifically, the twofilms were placed together with their film sides facing parallel andsandwiching the crystal, and the crystal was observed from one sidewhile irradiating a light source from the opposite side of the film. Thecrystal angle and position were changed and the non-single crystalportion was measured as a polycrystalline. The final polycrystallineportion volume was calculated, and the ratio of the total crystal volumeand the polycrystalline portion volume was determined as the incidenceof polycrystal formation. Crystals with an incidence of polycrystalformation of less than 30% were evaluated as ⊚, and those above 70% wereevaluated as ∘.

The evaluation results are shown in Table 2, which clearly shows that ifthe crucible inner surface is coated with glass-like carbon (GC) to athickness of at least 1.5 mm and the contact angle between the surfaceand water droplets is no greater than 100°, as in Experimental Examples6-8, the incidence of polycrystal formation is 30% or lower.

However, in cases where the glass-like carbon (GC) coating thickness wasless than 1 mm and the contact angle between the surface and waterdroplets exceeded 100°, as in Experimental Examples 9 and 10, theincidence of polycrystal formation was 70% or greater. This demonstratedthat the crucibles of Experimental Examples 6-8 allowed easier growth ofcalcium fluoride (CaF₂) single crystals compared to the crucibles ofExperimental Examples 9 and 10. TABLE 2 Contact angle between crucibleinner surface and GC coating water droplets thickness Evaluation Exp.Example 6  85° 2.0 mm ⊚ Exp. Example 7  90° 1.5 mm ⊚ Exp. Example 8 100°1.5 mm ⊚ Exp. Example 9 107° 1.0 mm ∘ Exp. Example 127°   0 mm ∘ 10

Experimental Example 11

The crucible 1 shown in FIG. 1 was prepared first. The crucible body 1A,cover member 1B and bottom member 1C were all constructed of high-puritycarbon (high-purity carbon by Nippon Carbon Co., Ltd.). The wall surface1K of the seed carrying section 1E was cylindrical, and the innerdiameter was 20 mm. The bottom face of the seed carrying section 1E wasflat and vertical with respect to the wall surface 1K. The wall surfaceof the starting material carrying section was also cylindrical, with aninner diameter of 250 mm. The curvature radius of the concave curvedplane 1J was 60 mm, and the curvature radius of the convex curved planewas 50 mm. The inner surface of the starting material carrying section1D and the inner surface of the seed carrying section were coated withglass-like carbon (Glass-like carbon coat by Nisshinbo Industries, Inc.)to an impregnating layer thickness of 1.0 mm, and the water dropletcontact angle was 90°.

The cover member 1B of this crucible 1 was removed, and a cylindricalseed S with a diameter of 10 mm and a length of 10 cm was loaded intothe seed carrying section 1E of the crucible 1. The material of the seedS used was calcium fluoride, and the seed S had a shape with flat edges.

The calcium fluoride starting material M was then loaded into thestarting material carrying section 1D. Next, the starting materialcarrying section 1D of the crucible body 1A was closed with the covermember 1B.

The pressure inside the vacuum VB furnace 2 was then reduced to below10⁻⁴ Pa, the heater 2A was heated to about 1400-1500° C., and thecrucible 1 was raised at an imperceptible speed of about 10 mm/h by theshaft 2B and held at the raised position for about 10 hours. During thatperiod, the lower part of the seed S was forcefully cooled through theheat transfer member 2D by cooling water circulating in the shaft 2Bfrom the inner conduit 2B1 to the outer conduit 2B2.

After the calcium fluoride starting material M melted, the crucible 1was lowered by the shaft 2B at an imperceptible speed of about 1.0 mm/hand was held at the lowered position in the vacuum VB furnace 2 forabout 5 hours. The melted calcium fluoride (CaF₂) starting material Mwas thus cooled to grow a single crystal along the crystal plane of the(111) orientation of the seed S.

Next, the solidified calcium fluoride (CaF₂) in the crucible 1 wascooled at a cooling rate of about 30° C./h by ON/OFF control of theheater 2A of the vacuum VB furnace 2.

Experimental Example 12

A crucible was prepared in the same manner as Experimental Example 1,except that the bottom face of the seed carrying section 1E was conical,and this crucible was used to grow a single crystal of calcium fluoridein the same manner as Experimental Example 11.

Evaluation of Crystal Quality

The single crystals of calcium fluoride obtained in ExperimentalExamples 11 and 12 were set between two Polar films by Edmund IndustrialOptics (color: gray, area: 15 in.×8.5 in., thickness: 0.29 mm) with thetwo film faces placed together facing parallel, the crystal was observedfrom the outside of one film while irradiating a light source from theoutside of the other film, and the crystal angle and position werechanged for measurement of the non-single crystal portion aspolycrystalline. The final polycrystalline portion volume wascalculated, and the ratio of the total crystal volume and thepolycrystalline portion volume was determined as the incidence ofpolycrystal formation. As a result, the incidence of polycrystalformation was below 30% in the single crystal of Experimental Example11, while the incidence of polycrystal formation was above 30% in thesingle crystal of Experimental Example 12. This indicated that thesingle crystal of Experimental Example 11 was of superior crystallinequality compared to the single crystal of Experimental Example 12. Thus,it was confirmed that a single crystal of satisfactory crystallinequality can be obtained by eliminating gaps between the bottom face ofthe seed and the bottom face of the seed carrying section.

Experimental Example 13

A crucible made of high-purity carbon was prepared with φ175 mm outerdiameter×325 mm height (φ10 mm inner diameter×100 mm height of the seedcarrying section), and thermocouples (JIS-Type B) with φ3.5 mmdiameter×80 mm length were mounted at three locations, 50 mm above thebottom end of the seed carrying section, 95 mm above the bottom end ofthe seed carrying section and near the starting material carryingsection.

FIG. 8 shows the relationship between the crucible position andthermocouple temperature, when the pressure inside the vacuum VB furnace2 was reduced to below 10⁻⁴ Pa while the crucible interior was empty,and with the crucible heated by the primary and secondary heaters. Thecrucible position 12 was initially at the position shown in FIG. 6, andthe crucible was lowered with time. The primary heater temperature 10was kept at 1500° C., and the secondary heater temperature 11 wasincreased to about 950° C. and then regulated. The temperatures at thethree locations of the crucible, specifically the temperature 13 of thethermocouple 50 mm above the bottom end of the seed carrying section,the temperature 14 of the thermocouple 95 mm above the bottom end of theseed carrying section and the temperature 15 of the thermocouple nearthe starting material carrying section, clearly are in correlation withthe primary heater temperature 10, secondary heater temperature 11 andcrucible position 12.

A high-purity calcium fluoride single crystal (φ10 mm diameter×100 mmlength, crystal orientation (111) in lengthwise direction, product ofHitachi Chemical Co., Ltd.) was loaded as the seed into the seedcarrying section of the crucible, and high-purity calcium fluoridepowder (product of Stella Chemifa Corp.) was loaded as the calciumfluoride starting material into the starting material carrying section.In addition, high-purity zinc fluoride powder (product of Stella ChemifaCorp.) was included as an additive with the calcium fluoride startingmaterial. The crucible position was in the lowermost state.

Next, the pressure inside the vacuum VB furnace was reduced to below10⁻⁴ Pa, the primary heater was heated to about 1500° C., and thecrucible position was raised at a speed of 10 mm/h and held at theraised position for about 10 hours. During this time, the temperature ateach position was measured by the thermocouples, and the primary heatermoving was terminated when the boundary position between the meltedportion and unmelted portion of the seed loaded in the seed carryingsection reached a position 50 mm above the bottom end of the seedcarrying section, at which time the crucible position was lowered at aspeed of no greater than 0.7 mm/h and held at the lowered positioninside the vacuum VB furnace for 5 hours. The inside of the vacuum VBfurnace was then cooled at a rate of 50-100° C./h. When the crucibleinterior temperature fell below 50° C., nitrogen was introduced into thevacuum VB furnace to atmospheric pressure, and the crystal in thecrucible was removed.

Experimental Examples 14-18

A calcium fluoride crystal was obtained for Experimental Example 14 inthe same manner as Experimental Example 13, except that the boundaryposition between the melted portion and unmelted portion of the seedloaded in the seed carrying section, upon termination of heating by theprimary heater and secondary heater, was at a position 80 mm above thebottom end of the seed carrying section. A calcium fluoride crystal wasalso obtained for Experimental Example 15 in the same manner asExperimental Example 13, except that the boundary position between themelted portion and unmelted portion of the seed loaded in the seedcarrying section, upon termination of moving by the primary heater andsecondary heater, was at a position 70 mm above the bottom end of theseed carrying section. A calcium fluoride crystal was also obtained forExperimental Example 16 in the same manner as Experimental Example 13,except that the boundary position between the melted portion andunmelted portion of the seed loaded in the seed carrying section, upontermination of moving by the primary heater and secondary heater, was ata position 60 mm above the bottom end of the seed carrying section. Acalcium fluoride crystal was also obtained for Experimental Example 17in the same manner as Experimental Example 13, except that the boundaryposition between the melted portion and unmelted portion of the seedloaded in the seed carrying section, upon termination of moving by theprimary heater and secondary heater, was at a position 40 mm above thebottom end of the seed carrying section. A calcium fluoride crystal wasalso obtained for Experimental Example 18 in the same manner asExperimental Example 13, except that the boundary position between themelted portion and unmelted portion of the seed loaded in the seedcarrying section, upon termination of moving by the primary heater andsecondary heater, was at a position 25 mm above the bottom end of theseed carrying section.

Experimental Examples 19 and 20

A calcium fluoride crystal was obtained for Experimental Example 19 inthe same manner as Experimental Example 13, except that the boundaryposition between the melted portion and unmelted portion of the seedloaded in the seed carrying section, upon termination of moving by theprimary heater and secondary heater, was at a position 85 mm above thebottom end of the seed carrying section. A calcium fluoride crystal wasalso obtained for Experimental Example 20 in the same manner asExperimental Example 13, except that the boundary position between themelted portion and unmelted portion of the seed loaded in the seedcarrying section, upon termination of moving by the primary heater andsecondary heater, was at a position 20 mm above the bottom end of theseed carrying section.

Each of the calcium fluoride crystals of Experimental Examples 13-20 wassandwiched between two polarizing films rotated at 90° from each other,and upon visual observation of light transmission with irradiation by anilluminator, the grain boundary was observed between the area with nolight transmission and the area with light transmission. If no grainboundary is observed, then the crystal is judged to be completely singlecrystal. After further working into a disc shape (φ150 mm×100 mmthickness) using a cutter and polishing with a mirror polisher, singlecrystallinity or polycrystallinity was reconfirmed in the same manner.The crystal orientation was confirmed by the X-ray/Laue method. Thejudgment was determined to be poor if the obtained crystal waspolycrystalline, or if the volume of the single crystal portion, i.e.the portion having the same crystal orientation (111) as the seed, wasless than 50 vol %, and the number of poorly formed crystals out of thenumber of tests (10) was evaluated. The conditions and results forExperimental Examples 13-20 are shown together in FIG. 9.

As is clear from FIG. 9, the most satisfactory calcium fluoride singlecrystal yields were obtained in Experimental Example 13 and ExperimentalExample 16, the second most satisfactory calcium fluoride single crystalyields were obtained in Experimental Example 15 and Experimental Example17, and the third most satisfactory calcium fluoride single crystalyields were obtained in Experimental Example 14 and Experimental Example18. In Experimental Examples 19 and 20, the crystal form of the calciumfluoride was polycrystalline, and single crystals could not be obtained.

1. A crucible for growth of a single crystal along the crystal plane ofa seed by melting and cooling an optical part material, characterized inthat the surface roughness of the inner surface of the crucible asmeasured by the maximum height method is no greater than Rmax 6.4s.
 2. Acrucible according to claim 1, wherein the crucible inner surface iscomposed of glossy glass-like carbon.
 3. A crucible according to claim1, wherein the crucible is composed of carbon as the material.
 4. Acrucible for growth of a single crystal along the crystal plane of aseed by melting and cooling an optical part material, characterized inthat a tapered cone surface is formed between the starting materialcarrying section in which the starting material of said optical partmaterial is loaded and the seed carrying section in which said seed isloaded, the wall surface of said starting material carrying section issmoothly connected to the cone surface via a concave curved plane, andsaid cone surface is smoothly connected to the wall surface of the seedcarrying section via a convex curved plane.
 5. A crucible for growth ofa single crystal along the crystal plane of a seed by melting andcooling an optical part material, characterized in that the cone angleof the tapered cone surface formed between the starting materialcarrying section in which the starting material of said optical partmaterial is loaded and the seed carrying section in which said seed isloaded is set in a range between 95° and 150°.
 6. A crucible for growthof a single crystal along the crystal plane of a seed by melting andcooling an optical part material, characterized in that the contactangle between the crucible inner surface and water droplets is nogreater than 100°.
 7. A crucible according to claim 6, wherein thecrucible inner surface is composed of glass-like carbon.
 8. A crucibleaccording to claim 7, wherein the portions other than the crucible innersurface are composed of carbon as the material.
 9. A crucible for growthof a single crystal along the crystal plane of an optical part materialseed by melting and cooling the optical part material, characterized bycomprising a starting material carrying section in which the startingmaterial of said optical part material is loaded, and a seed carryingsection in which said seed is loaded, wherein the bottom of said seedcarrying section has a shape matching the edge of said seed.
 10. Acrucible according to claim 9, wherein the edge of said seed has an edgeface and a side connected to said edge face, while the bottom of saidseed carrying section has a bottom face and a wall surface which isconnected to said bottom face and matches the side of said seed, whereinboth said edge face and said bottom face are flat surfaces.
 11. Acrucible according to claim 9, wherein said optical part material iscalcium fluoride.
 12. A single crystal growth method whereby a singlecrystal of an optical part material is grown using a crucible accordingto claim 9, characterized by comprising a seed loading step in which aseed having an edge with a shape matching the bottom of said seedcarrying section is loaded as the seed in said seed carrying section ofsaid crucible, a starting material loading step in which said opticalpart material is loaded as the starting material in said startingmaterial carrying section, and a growth step in which a single crystalof the optical part material is grown along the crystal plane of saidseed by melting and cooling said starting material in said crucible. 13.A crucible for growth of a single crystal of an optical part material,characterized by comprising a closed-bottom seed carrying sectionextending in the vertical direction, in which a seed is loaded, astarting material carrying section in which the single crystal of saidoptical part material is loaded, which is situated above said seedcarrying section and is connected to said seed carrying section, andtemperature detecting means for detection of the internal temperature ofsaid seed carrying section.
 14. A crucible according to claim 13,characterized in that said temperature detecting means is athermocouple, and said thermocouple is situated at a position near thewall surface of said seed carrying section.
 15. A crucible according toclaim 14, characterized in that a plurality of thermocouples are usedand are situated at mutually separated positions in the verticaldirection.
 16. A crucible according to claim 15, characterized in thatone of the two thermocouples is situated at a position at a height abovethe bottom end of said seed carrying section corresponding to 25-50% ofthe depth of said seed carrying section, while the other is situated ata position at a height above the bottom end of said seed carryingsection corresponding to 60-80% of the depth of said seed carryingsection.
 17. A crucible according to claim 13, wherein said optical partmaterial is calcium fluoride.
 18. A single crystal growth methodcharacterized by comprising a step of preparing a crucible according toclaim 13, a step of loading the seed of the optical part material intosaid seed carrying section, a step of loading the starting material forthe single crystal of said optical part material into said startingmaterial carrying section, a step of situating said crucible in acrystal growth furnace heated in such a manner that the interior has aspecified temperature gradient in the vertical direction, and heatingsaid crucible so that the starting material carried in said startingmaterial carrying section and the seed carried in said seed carryingsection gradually melt from top to bottom, a step of detecting theinternal temperature of said seed carrying section by said temperaturedetecting means during heating of said crucible, and a step ofterminating the heating and commencing the cooling for growth of asingle crystal when, based on the internal temperature of said seedcarrying section detected by said temperature detecting means, theboundary position between the melted portion and unmelted portion of theseed carried in said seed carrying section is judged to be between afirst position which is at a prescribed height above the bottom end ofsaid seed carrying section and a second position which is at aprescribed height above said first position.
 19. A single crystal growthmethod according to claim 18, wherein said first position is a positionat a height above the bottom end of said seed carrying sectioncorresponding to 25% of the depth of said seed carrying section, andsaid second position is a position at a height above the bottom end ofsaid seed carrying section corresponding to 80% of the depth of saidseed carrying section.
 20. A crucible according to claim 1, wherein saidoptical part material is calcium fluoride.
 21. A crucible according toclaim 4, wherein said optical part material is calcium fluoride.
 22. Acrucible according to claim 5, wherein said optical part material iscalcium fluoride.
 23. A crucible according to claim 6, wherein saidoptical part material is calcium fluoride.